1. Field of the Invention
The invention generally relates to memory technology. In particular, the invention relates to non-volatile magnetic memory.
2. Description of the Related Art
Computers and other digital systems use memory to store programs and data. A common form of memory is random access memory (RAM). Many memory devices, such as dynamic random access memory (DRAM) devices and static random access memory (SRAM) devices are volatile memories. A volatile memory loses its data when power is removed. For example, when a conventional personal computer is powered off, the volatile memory is reloaded through a boot up process. In addition, certain volatile memories such as DRAM devices require periodic refresh cycles to retain their data even when power is continuously supplied.
In contrast to the potential loss of data encountered in volatile memory devices, nonvolatile memory devices retain data for long periods of time when power is removed. Examples of nonvolatile memory devices include read only memory (ROM), programmable read only memory (PROM), erasable PROM (EPROM), electrically erasable PROM (EEPROM), flash memory, and the like. Disadvantageously, conventional nonvolatile memories are relatively large, slow, and expensive. Further, conventional nonvolatile memories are relatively limited in write cycle capability and typically can only be programmed to store data about 10,000 times in a particular memory location. This prevents a conventional non-volatile memory device, such as a flash memory device, from being used as general purpose memory.
An alternative memory device is known as magnetoresistive random access memory (MRAM). An MRAM device uses magnetic orientations to retain data in its memory cells. Advantageously, MRAM devices are relatively fast, are nonvolatile, consume relatively little power, and do not suffer from a write cycle limitation. A pseudo spin valve (PSV) MRAM device uses an asymmetric sandwich of the ferromagnetic layers and metallic layer as a memory cell, and the ferromagnetic layers do not switch at the same time.
The asymmetric sandwich of a PSV MRAM includes a xe2x80x9chard layerxe2x80x9d that stores data and a xe2x80x9csoft layerxe2x80x9d that switches or flips to allow data to be stored and read in the hard layer. When operating as intended, the soft layer switches before the hard layer. The earlier switching of the soft layer advantageously inhibits switching of the hard layer, which then results in a higher write threshold for a PSV MRAM than for a spin valve MRAM.
One problem with conventional PSV MRAM devices is that the magnetization of the soft layer is not well controlled. A soft layer that fails to switch at a relatively low applied magnetic field can result in a PSV MRAM device that undesirably behaves as a spin valve rather than a PSV. This reduces the write threshold and can result in corrupting the stored data during a read operation. To protect PSV MRAM devices from data corruption, the fields generated during read operations are maintained to relatively low levels, which results in relatively low repeatability and cyclability of writing to and reading from memory cells.
Embodiments of the invention solve these and other problems by stabilizing the soft layer of a pseudo spin valve (PSV). Embodiments of the invention include a layer of antiferromagnetic material (AFM), which stabilizes the magnetization of the thin layer. The stabilization of the soft layer of the PSV provides PSV MRAM devices with relatively good repeatability and cyclability.
Embodiments of the invention include an antiferromagnet in a magnetic memory cell. An antiferromagnetic layer can be formed adjacent to a soft layer in an MRAM on a side of the soft layer that is opposite to a hard layer of the MRAM. One arrangement further includes an additional interlayer of non-antiferromagnetic material between the antiferromagnetic layer and the soft layer.
The antiferromagnetic material (AFM) is formed adjacent to or near to the soft layer of the PSV. The layer of AFM should be formed on a side of the soft layer that is opposite to a side with a hard layer of the PSV. In addition, an amount of coupling between the soft layer and the AFM layer should be sufficiently low enough to allow the soft layer to switch at a lower magnetic field than the hard layer, thereby maintaining a relatively wide spread between the strength of a magnetic field used in a read operation and the strength of a magnetic field used in a write operation.
One embodiment of the invention includes an antiferromagnetically stabilized pseudo spin valve (ASPSV) in a magnetic random access memory (MRAM). The ASPSV includes a hard layer of ferromagnetic material, a soft layer of ferromagnetic material, a spacer layer of non-ferromagnetic material disposed between the hard layer and the soft layer; and an antiferromagnetic layer disposed adjacent to the soft layer. The antiferromagnetic layer should also be disposed on a side of the soft layer that is opposite to the hard layer. The antiferromagnetic layer can be formed from an alloy of manganese, such as from ferro manganese (FeMn).
Another embodiment of the invention includes an antiferromagnetically stabilized pseudo spin valve (ASPSV) in a magnetic random access memory (MRAM) with an AFM interlayer. The ASPSV includes a hard layer of ferromagnetic material adapted to store data in a magnetic orientation, a soft layer of ferromagnetic material adapted to switch orientation to allow data to be read from the hard layer, a spacer layer of non-ferromagnetic material disposed between the hard layer and the soft layer, an antiferromagnetic layer disposed on a side of the soft layer that is opposite to the hard layer, and the AFM interlayer. The AFM interlayer is disposed between the soft layer and the antiferromagnetic layer. The AFM interlayer can be formed from a variety of materials, but should not be formed from an antiferromagnetic material. Suitable materials for the AFM interlayer include iridium (Ir), copper (Cu), ruthenium (Ru), chromium (Cr), and aluminum (Al). The AFM interlayer can be relatively thin, such as about a monolayer in thickness.
Another embodiment of the invention includes a method of stabilizing a pseudo spin valve (PSV). The method includes providing a magnetoresistive sandwich that includes a soft layer and a hard layer, and forming an antiferromagnetic layer on the magnetoresistive sandwich near to the soft layer and on a side of the soft layer that is opposite to the hard layer. The antiferromagnetic layer can be formed adjacent to the soft layer, or a AFM interlayer can also be formed between the soft layer and the antiferromagnetic layer.